Can I change the speed of an image sequence?
It is possible by using drivers:


Add an Empty object somewhere;
In its Object panel, choose one Transform channel (such as Z Scale), type in #frame, then hit Enter to confirm. It will add a quick driver for the empty, make the Z scale value the same as the current frame number (e.g. If the current frame is 6, then the scale value will be driven to 6 automatically).
In Cycles Material node editor, move the mouse cursor over the Offset in Image Texture node, hit D to add a driver.
Open Graph Editor, switch to Driver mode, keep that node selected and you'll see the driver added. Set a variable based on the Z scale property of that Empty object. Type the expression like this: variable_name * speed_fac.


For example, in the example below, I used the variable name as var by default, and use 3 as speed factor, which means to make it playing 3X speed up.





NOTE:


It's recommended to control node-related drivers by object transform, which is the tricky part, due to the current dependency limit on nodes. 
If The warning message occurs (as below), click Reload Trusted.